Problem: Some cars were parked in the grocery store parking lot. 20 more cars park at the grocery store. Now there are 21 cars in the parking lot. How many cars were parked at the grocery store at the beginning?
Explanation: The number of cars parked at the store at the beginning is the difference of the number parked now and the number that came and parked earlier. The difference is $21 - 20$ cars. $21 - 20 = 1$ car.